(IDEX Online) - Tiffany & Co says it has stopped sourcing diamonds mined in Russia, which are said to account for a third of its general supply.It's the second major US jeweler to implement a boycott on newly-mined stones from Russia in response to its invasion of Ukraine. Signet said last week it would not accept precious metals of diamonds from Russia, even if suppliers were not subject to sanctions in their own countries.US sanctions prohibit the importation of rough stones from Russia, but allow polished gems because they are deemed to have been substantially transformed.Tiffany says the ban on Russian stones was brought in on 21 March and covers all but melee diamonds - under 0.18-cts - which are harder to track. Rough diamonds typically take three months to work their way through the supply chain, so some stones mined in Russia after the invasion may still reach customers."Tiffany & Co. has been a leader in championing ethical sourcing practices for over 20 years. These bold changes reflect our continued effort to ensure that we set the bar high not only for ourselves, but for the entire industry," said CEO Anthony Ledru. "We are deeply concerned about the ongoing crisis in Ukraine."